1. Field of Invention
The present invention relates in general to valve mechanisms of an internal combustion engine, and more particularly to the valve mechanisms of a type that varies a valve lift characteristic of an intake or exhaust valve of the engine in accordance with an operation condition of the engine.
2. Description of Related Art
Hitherto, various valve mechanisms of an internal combustion engine have been proposed and put into practical use. One of them is disclosed in Japanese Laid-open Patent Application (Tokkai) 2001-55915. For clarifying the present invention, the valve mechanism of the application will be briefly described in the following.
The valve mechanism is incorporated with an internal combustion engine of a type having two intake valves for each cylinder, and comprises a drive shaft driven synchronously by a crankshaft of the engine, an eccentric drive cam mounted to the drive shaft to rotate therewith, and a cylindrical hollow cam shaft concentrically disposed about the drive shaft. To this cam shaft, there are mounted a pair of swing cams for the intake valves of the engine. To the swing cams, there is transmitted a rotation force (viz., torque) of the eccentric drive cam through a movement transmitting mechanism, so that the intake valves are actuated to open and close through respective valve lifters. The cam shaft is rotatably supported, at a journal portion thereof provided between the two swing cams, by a cam shaft bearing portion which includes a shaft bearing recess formed in an upper surface of a cylinder head of the engine and a bearing bracket.
The movement transmitting mechanism comprises a rocker arm that is rotatably held by a control shaft through a control cam, a link arm that has one end rotatably disposed about the eccentric drive cam and the other end pivotally connected to an end of the rocker arm, and a link rod that has one end pivotally connected to the other end of the rocker arm and the other end pivotally connected to a leading end portion of a cam nose portion of one of the swing cams.
To a cylindrical outer surface of the control shaft, there is mounted an eccentric control cam to rotate therewith. This eccentric control cam is rotatably received in a holding bore formed in a generally intermediate portion of the rocker arm, so that a fulcrum of the rocker arm is displaced in accordance with rotation of the eccentric control cam. With the displacement of the fulcrum, the contact point of a cam surface of each swing cam relative to an upper surface of the valve lifter moves thereby to continuously vary the lift degree of each intake valve.
By varying or adjusting the valve lift characteristic in accordance with an operation condition of the engine, fuel consumption, output performance and the like are improved.
However, due to an inherent construction, the above-mentioned known valve mechanism tends to exhibit a non-negligible inclination or lurching of the cam shaft particularly when the intake valves are under their opening stroke. As is known, such inclination brings about a development of uneven wearing of bearing portions of the cam shaft.
Accordingly, it is an object of the present invention to provide a valve mechanism of an internal combustion engine, which is free of the above-mentioned drawback.
According a first aspect of the present invention, there is provided a valve mechanism of an internal combustion engine, which comprises a drive shaft synchronously rotated with a crankshaft of the engine; a supporting shaft; a cylindrical hollow cam shaft rotatably disposed about the supporting shaft keeping a first cylindrical clearance therebetween, the cam shaft being formed with at least one cam that actuates an engine valve for opening and closing the same; a bearing which bears a given portion of the cam shaft keeping a second cylindrical clearance therebetween; and a movement transmitting mechanism which transmits a torque of the drive shaft to the cam shaft while converting a rotation movement of the drive shaft to a swing movement of the cam shaft, the thickness of the first cylindrical clearance being smaller than that of the second cylindrical clearance.
According to a second aspect of the present invention, there is provided a valve mechanism of an internal combustion engine which has two engine valves for each cylinder which are biased in a closing direction by valve springs, the valve mechanism comprising a supporting shaft; a swing member having a through bore, the swing member being swingably disposed on the supporting shaft having the supporting shaft received in the through bore, the swing member operating the engine valves to open against a biasing force of the valve springs when swung; and a drive mechanism that applies a drive force to the swing member to effect the swing movement against the biasing force of the valve springs, the drive mechanism being constructed to apply the drive force to one end portion of the swing member with respect to an axial direction of supporting shaft, wherein, when the engine valves are maximally opened by the swing member, a distance from a second diametrically center position of the through bore of the swing member to a head member of the corresponding engine valve is longer than a distance from a first diametrically center position of the through bore of the swing member to a head member of the corresponding engine valve, the first diametrically center position being in the vicinity of the one end portion of the swing member.
According to a third aspect of the present invention, there is provided a valve mechanism of an internal combustion engine having first and second engine valves for each cylinder, which comprises a drive shaft rotated synchronously together with a crankshaft of the engine; a cylindrical hollow cam shaft rotatably disposed about the drive shaft keeping a first cylindrical clearance therebetween, the cam shaft being formed with first and second cams that respectively actuate the first and second engine valves for opening and closing the same; a bearing which bears a given portion of the cam shaft keeping a second cylindrical clearance therebetween, the given portion being positioned between the first and second cams; and a movement transmitting mechanism which transmits a torque of the drive shaft to the cam shaft while converting a rotation movement of the drive shaft to a swing movement of the cam shaft, the movement transmitting mechanism being arranged to apply the torque to one axial end of the cam shaft where the first cam is provided, wherein the thickness of the first cylindrical clearance is smaller than that of the second cylindrical clearance.